1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device which uses an MIM (metal-insulator-metal) device as a nonlinear resistance thereof and which display device is applicable to high capacity flat panel display devices for OA (office automation) and TV, etc.
Also, the present invention relates to a matrix display device, especially to an active matrix display device which is applicable to flat panel display devices such as a liquid crystal display device, etc.
2. Description of the Related Art
An active matrix type liquid crystal display device in gereral comprises a pair of insulator substrates between which a liquid crystal layer is constituted wherein a nonlinear resistance device is connected in series to each of pixels of at least one of the substrates. Such a nonlinear resistance device usually comprises an MIM device.
An MIM device comprises an insulator substrate made from glass or the like, a lower electrode made from a metal such as Ta, Al, Ti etc., an insluation film comprising a transparent electrode made from the metal oxide of lower electrode metal or SiO.sub.x, SiN.sub.x etc., and an upper electrode made from a metal such as Al, Cr etc. disposed in this order on the substrate.
An example of the MIM device in which a metal oxide is used as the insulation film is disclosed in Japanese Patent Application Laying Open (KOKAI) Nos. 57-196589, 61-232689 and 62-52333. The metal oxide of this MIM device is formed by anodic oxidizing or thermal oxidizing of the lower electrode, which makes the producing process of the device very complicated. Besides, the process requires a thermal treatment at a high temperature (the anodic oxidizing process also requires the thermal treatment for removing impurities.). Further, in accordance with the process, it is not easy to accurately control the film quality and the film thickness so that uniform products are not always obtained. Further, the substrate has to be made from a heat-resistant material and the insulation film has to be made from a metal oxide having a stable characteristic, which limits the materials to be selected and the characteristic of the device can not be easily changed and as a result the freedom of design is narrowed. This means that it is difficult to realize a liquid crystal display device utilizing the MIM device in which the display device fully satisfies the requirements from the user. Also, due to the low controllability of the film of the MIM device mentioned above, the device characteristics of current (I) and voltage (V), especially symmetry of the I-V characteristic is impaired so that the ratio (I.sub.- /I.sub.30) of the minus biassed current I.sub.- with respect to the plus biassed current I.sub.+ becomes unstable. Also, when the MIM device is used as an element of the liquid crystal display (LCD), it is necessary that the ratio of the liquid crystal capacity (C.sub.LCD) with respect to the MIM capacity (C.sub.MIM) is more than 10. Therefore, the MIM capacity is desirably small. However, the metal oxide film has a large dielectric constant, which makes the element capacity large. Therefore, it becomes necessary to process the device to form a structure having a small surface area so as to reduce the device capacity. Also, in this case, at the time of the rubbing process for sealing the liquid crystal in the device, the insulation film is sometimes mechanically damaged, which reduces the throughput of the device.
Besides, with respect to the large capacity device having a large display area, there are problems of short circuit between the devices, snapping of wires and misorientation of the liquid crystal due to dust and defective pixels or electrodes, which degrade the display quality of the device.
In order to obviate the above mentioned problems, Japanese Patent Application Laying Open (KOKAI) No. 62-59927 discloses a display device in which an image to be displayed is divided into at least two parts each of which is connected to a switching means composed of an MIM device. However, by dividing the image, the capacity ratio mentioned above may become undesirable. Therefore, it is desirable, from the aspect of this point too, that the MIM capacity be small as possible.
On the other hand, with respect to the MIM device comprising the insulation film of SiO.sub.x or SiN.sub.x as disclosed in Japanese Patent Application Laying Open (KOKAI) No. 61-275819, the insulation film is formed by a gas or vapor phase process such as a plasma CVD method or a sputtering method which is conducted at a temperature more than 300.degree. C., which requires a heat resistant substrate which is expensive. Also, it is not easy to obtain a film of even thickness and uniform quality owing to the temperature distribution over the large area of the display substrate. Also, the insulation film is made from an amorphous substance which is easy to change the characteristic thereof, which causes problems of optical degradation of the device and change of the photoelectricity (resistance change according to the luminous intensity). Therefore, also in this case, the design freedom of the device is narrowed. Also, dust particles are generated in the gas phase at the time of film forming process, which causes pin holes of the film and also reduces the throughput of the device.
Also, the insulation film used in the known MIM device is not fully sufficient with regard to the dielectric breakdown voltage and the threshold voltage.
Also, the present inventors have proposed an MIM device comprising an insulation film made from a hard carbon film (i-type carbon) which is very thin (about 20 to 100 .ANG.). The conductive system using this thin film is based on the tunnel conductivity of the film. Such a thin film is suitable for use as a superthin film element such as a high speed switching element and a tunnel light emission element. However, when the film is applied to the liquid crystal display device, it is desirable that the film be thick from the standpoint of dielectric breakdown voltage, throughput (productivity), uniformity of the element characteristic and/or threshold voltage.
Considering the above-mentioned points, it is proposed to connect two MIM devices in series for one pixel in Japanese Patent Application Laying Open (KOKAI) Nos. 61-284728 and 62-69239. However, the insulation film of the MIM device is formed on the temperature condition of 200.degree. to 300.degree. C. or more, which limits the substrate material. Therefore, the dielectric constant of the substrate has to be large, which impedes making the MIM device surface sufficiently small.
An active matrix display device comprises a metal-insulator-metal device (MIM device). A typical MIM device comprises, as disclosed in Japanese Patent Application Laying Open (KOKAI) No. 62-62333, a lower electrode made from Ta, an insulation film made from Ta.sub.2 O.sub.5 (anodic oxidized film) and an upper electrode made from Cr or Cr/ITO. Heat treatment of about 300.degree. to 500.degree. C. is conducted to obtain a symmetric polarity of the device.
Also, another example of the MIM device is disclosed in Japanese Patent Application Laying Open (KOKAI) No. 61-260219 in which the lower electrode is made from ITO and the insulation film is made from SiN.sub.x which is formed by a plasma CVD method and the upper electrode is made from Cr.
Also, Japanese Patent Application Laying Open (KOKAI) No. 63-187279 discloses a matrix display device in which the above-mentioned MIM devices are used in such a way that each pixel is connected to two scanning lines through two MIM devices.
However, the matrix display device comprising the known MIM devices disclosed in the publications has problems of wire snapping and short circuit, which impairs the display quality of the device, since the MIM device of the publications has problems as mentioned below.
First, when the insulation film of the MIM device is formed by anodic oxidizing, it is not possible to desirably control or change the characteristic of the MIM device since the insulation film material is limited to the metal oxide formed by anodic oxidizing of the lower electrode metal.
Second, the substrate material is limited to a heat resistant material since the device has to be treated at a temperature of 300.degree. to 500.degree. C.
Third, the ratio of MIM capacity with respect to the liquid crystal capacity has to be smaller than 1/10. Therefore, when the device is used as a switching element of the liquid crystal device, it becomes necessary to make the device surface small since the dielectric constant of the substrate is high, which requires a fine processing of high accuracy.
Also, with regard to the MIM device comprising the insulation film of SiN.sub.x, the film is formed at a high temperature of about 300.degree. C., which limits the material of the substrate, as in the case of above-mentioned MIM device. Besides, pin holes are eaasy to be formed in the device due to dust particles, which reduces the throughput of the products.